Nasa Picks "juno" As Next New Frontiers Mission Options

[Guest_BruceMoomaw_*]

http://www.nasa.gov/home/hqnews/2005/jun/H...rontiers_2.html

Yeah, I know it ain't Saturn, but we don't seem to have any proper slot for Jovian news -- including yesterday's totally unexpected announcement that Amalthea's density is so low as to suggest that it's a highly porous ice object; maybe a captured Kuiper Belt Object reduced to rubble by infalling meteoroids. As Jason Perry says, this might explain those previously mysterious light-colored patches on Amalthea -- they may be its underlying ice, exposed by impacts that punched through the layer of sulfur spray-painted onto it by Io.

Scott Bolton has been pretty talkative to me already about the design of Juno. It certainly won't be as good in the PR department as Galileo or Cassini, but it DOES carry a camera -- as much for PR as for Jovian cloud science, according to Bolton. And since the latitude of periapsis of its highly elliptical orbit will change radically during the primary mission, I wonder if they might be able to set up at least one close photographic flyby of Io and/or Amalthea? (I believe, by the way, that this selection is a bit ahead of schedule -- and it certainly indicates that NASA's science program under Griffin won't be a complete slave to Bush's Moon-Mars initiative.)

[Guest_BruceMoomaw_*]

(1) "Imagine the way Halley's Comet's orbit intersects the Earth's (assuming the two orbits were nearly coplanar). They intersect in two places. On some passes, a close encounter will happen at one intersection, on some, at the other. On many, no encounter at all. Of course, there's no design behind the natural case. Clever choice of orbital period could allow close encounters to occur at both locations, on different orbits."

Ah, I see -- you're assuming that most of the Observer's orbit would be OUTSIDE Io's orbit, whereas I was assuming that it would be in a polar orbit INSIDE Io's orbit (like Juno) and intercept Io only at each of its apoapses. Put it in a polar orbit mostly outside Io's orbital distance -- and put its periapse at one of Jupiter's poles, so that it can intercept Io either while going inward or coming back out -- and you can indeed have it intercept Io's orbit at two different spots. I need to talk with Spencer and Smythe as to whether this is the sort of orbit they might have in mind for the Observer. (I lack a good visual imagination when it comes to orbits -- or, indeed, anything else.)

By the way, one fact I forgot to mention: in response to Van Kane's other question, Galileo only ventured slightly inside Io's orbit during each of its Io flyby orbits.

(2) "If Io were only to be observed from afar, then a perijove near Ganymede would suffice to save a whole lot of delta-v."

True -- but any orbiter can do that (Europa Orbiter will regularly do it from as close as Europa's orbit). The central goal of Io Observer will be to take repeated close-up looks at Io -- so the problem, once again, is whether there's any practical way to combine its mission with that of the Ganymede-Callisto Observer (hereafter called, by me, the GC Observer).

(3) "Interesting, and Saturn would probably serve much the same scientific purposes, if the two giants ended up with similar ratios of raw material."

This is going back a ways; but, given the difficulty of a Jupiter entry, back in the early 1970s serious consideration was given to making Saturn, rather than Jupiter, the target of the first giant-planet entry probe. (Who knows? We might have ended up sending Galileo to Saturn instead, and then ended up bitching even more about the consequences of its high-gain antenna failure.)

[JRehling]

Ah, I see -- you're assuming that most of the Observer's orbit would be OUTSIDE Io's orbit, whereas I was assuming that it would be in a polar orbit INSIDE Io's orbit (like Juno) and intercept Io only at each of its apoapses.  Put it in a polar orbit mostly outside Io's orbital distance -- and put its periapse at one of Jupiter's poles, so that it can intercept Io either while going inward or coming back out -- and you can indeed have it intercept Io's orbit at two different spots.  I need to talk with Spencer and Smythe as to whether this is the sort of orbit they might have in mind for the Observer.  (I lack a good visual imagination when it comes to orbits -- or, indeed, anything else.)

Delta-v could vary sharply from one of these possibilities to another, and in the absence of some mission-critical geometry need, it would, I expect, be a powerful factor. Orbiting close to Jupiter costs delta-v. Inclined orbits cost delta-v. I guess that something in a mainly-outside, coplanar orbit would win in every way but one, which is that it would probably take more radiation/pass, and thus live a shorter life. But if the point is to have a long duration of temporal coverage, a longer orbit with increased apojove would provide that, at the cost of time resolution (more flybys).

(2)  "If Io were only to be observed from afar, then a perijove near Ganymede would suffice to save a whole lot of delta-v."

True -- but any orbiter can do that (Europa Orbiter will regularly do it from as close as Europa's orbit).  The central goal of Io Observer will be to take repeated close-up looks at Io -- so the problem, once again, is whether there's any practical way to combine its mission with that of the Ganymede-Callisto Observer (hereafter called, by me, the GC Observer).

I've looked at some Cassini images of saturnian moons at typical Ganymede-to-Io distances, and the resolution is really not bad. Europa-to-Io would be better. What is the point of such a mission: to track lava flows at decameter resolutions?